Conventionally, a heat exchanger is made of a three-layer clad material of an aluminum alloy, including a brazing material layer, a core material layer and a sacrifice material layer. In the heat exchanger, a flat tube is formed by bending the three-layer clad material. Component parts of the heat exchanger including a plurality of the tubes are assembled, and then the components parts are brazed by heating, so as to form the heat exchanger.
Recently, it is necessary to reduce the thickness of the component parts in order to reduce the entire weight of the heat exchanger. With this, in order to reduce the thickness of the component parts while the mechanical strength and the durability of the heat exchanger are maintained, the material for forming the tube is required to have a high strength. For example, JP 2006-131923A describes regarding an aluminum alloy clad material in which magnesium (Mg) is added in a core material layer in order to increase the strength.
However, when Mg is added in the core material layer, the Mg in the core material layer diffuses to the surface of the brazing material layer, and reacts with flux in a brazing, thereby forming a high melting-point chemical composition such as MgF2. Thus, the flux component is reduced in the brazing, thereby reducing the brazing performance.
In the clad material of the aluminum alloy described in JP 2006-131923A, a middle material layer is formed between the core material layer and the brazing material layer, so as to reduce diffusion of the Mg from the core material layer to the brazing material layer by using the middle material layer. However, in this case, the clad material is configured by four-layer structure of the brazing material layer, the middle material layer, the core material layer and the sacrifice material layer, and thereby the product cost of the heat exchanger is increased.